cp: Duplicate source error message should specify the type of file.

[matrixhead]

When a source directory is specified more than once, the error message should clearly indicate that it’s a directory, not a file.

Example:

mkdir b; mkdir d;
cp -r b b d/

GNU’s cp produces the following message:

warning: source directory 'b' specified more than once

Whereas uu-cp outputs:

warning: source file 'b' specified more than once

[cakebaker]

Fixed by #6825